Commitments (Standby and Commercial Letters of Credit, Conditional Commitments Issued to Guarantee the Performance of a Customer to a Third Party) (Detail) - Financial Standby Letter of Credit $ in Millions	Sep. 30, 2017 USD ($)
Line of Credit Facility
Commitments	$ 2,401
Less Than One Year From The Balance Sheet Date
Line of Credit Facility
Commitments	1,198	[1]
More than One and within Five Years from Balance Sheet Date
Line of Credit Facility
Commitments	1,155	[1]
More than Five Years from Balance Sheet Date
Line of Credit Facility
Commitments	$ 48

[1] ( a) Includes $ 1 2 and $ 2 issued on behalf of commercial customers to facilitate trade payments in U.S. dollars and foreign currencies which expire less than 1 year and between 1 - 5 years, respectively.